In the last years laser metal forming (LMF) has been introduced in industrial manufacturing. Laser metal forming is a process where a high power laser locally melts a focussed stream of metal powder or a metal wire onto a substrate. In this way material can be added to the underlying part. The method is suitable for controlled material build-up and the laser generated parts are characterised by a dense microstructure which is usually free of pores.
Laser metal forming has been recently used for commercial manufacturing of superalloys due to its attractive potential for repair of locally damaged or worn parts. Indeed, it is possible to selectively add material at desired locations and to re-establish the full functionality of a component. It is clear that laser repair technology is particularly attractive for the refurbishment of expensive parts that are affected by local damage or local mechanical wear. Turbine blades and vanes are typical examples.
However, the process is complicated when single-crystal components have to be refurbished. Single crystal blades and vanes can be found in the most heavily loaded rows of modern gas turbines (first or high pressure row). Their mechanical integrity relies on the particular properties due to single-crystal microstructure and the absence of grain boundaries. Reconditioning of such components is only feasible if the single crystal microstructure can be maintained during the repair process.
So far, several patents have been issued for the laser metal forming process. The basic principle is described in EP-A1-0 558 870, DE-C1-199 49 972, U.S. Pat. Nos. 5,837,960, 5,622,638 or 4,323,756. During laser metal forming substrate material is locally molten and powder (or wire) is injected into the melt pool with a suitable powder (or wire) feeder mechanism. After a certain interaction time (which is determined by the laser spot size and the relative movement between laser and substrate) the molten material resolidifies leading to material build-up on the substrate. The process carries the particular advantage that, being numerically controlled, new design can be created offline and subsequent relatively quickly realised as prototype components. Processing occurs on a part-by-part basis, which, in great contrast to casting technology, ultimately gives the possibilities of reducing the batch size to just one component. A range of materials may be deposited by the same process so that specialised oxidation, wear or corrosion resistant regions may be formed as different parts of a functionally graded component. However, there are serious limitations, which limit the applicability of this otherwise useful process. Firstly, control over the deposited material, though thoroughly adequate for predominantly surface-based operations, is difficult to maintain as deposits become large and extensive. A second barrier to the metal forming of large monoliths is simply that the mass deposition rates, currently available in the art of epitaxial laser metal forming, would make the formulation of such artefacts an extremely time consuming operation. The fabrication of a large component would therefore best be achieved by commencing with a basic single crystal preform or blank, and modifying its shape by way of controlled addition of SX material. Thirdly, the powder stream may not be directed in all places it would be desired, because the proximity of the edges of other parts of the component obstruct the gas/power stream and make the process enviable. Such a feature, in which it is impossible to conduct epitaxial laser metal-forming is e.g. a large gap or crack. This limits the usefullness of the process in certain repair and modification operations.
Such a feature, in which it is impossible to conduct epitaxial laser metal-forming is e.g. a large gap or crack. This limits the usefulness of the process in certain repair and modification operations.
On the other hand other methods are generally known for repairing high temperature superalloys: U.S. Pat. No. 5,732,467 discloses a method of repairing cracks on the outermost surface of an article having a directionally oriented micro-structure and a superalloy composition. The repairing is done by coating the cleaned crack surface with a material featuring the same material composition as said article. Thereby the coated crack surface is subjected to an elevated temperature and isostatic pressure over a period of time sufficient to repair the crack surface without changing the crystalline microstructure of the parent article.
In addition, a number of alternative methods of brazing for repairing cracks or gaps are known. U.S. Pat. No. 5,666,643 discloses a braze material for repairing an article, in particular components made from a cobalt and a nickel-base super-alloy, such as gas turbine engine parts. The braze material is composed of particles featuring a high melting temperature which are distributed within the a braze alloy. These particles could be of single crystal, directionally solidified, or equiaxed microstructure. But, even if particles featuring a single crystal structure are used, the structure of the repaired crack as a whole due to the braze alloy differs with respect to material properties from the single-crystal structure of the base material which leads to weakness problems of the brazed joint. This is especially valid for cracks located at stress concentrations.
The same problem occurs with the repair methods disclosed in U.S. Pat. Nos, 4,381,944 or 5,437,737 where a braze alloy and a filler material are used at the same time to increase the strength of the brazed joint. Another method of repairing sintering is disclosed in U.S. Pat. No. 5,156,321.